a) Field of the Invention
The present invention relates to a zoom lens system having a high vari-focal ratio covering a range of wide angles and to be used mainly with photographic cameras.
b) Description of the Prior Art
As zoom lens systems having high vari-focal ratios covering ranges of wide angles, there are known the zoom lens systems consisting of a plural number of lens units including a first lens unit which is arranged on the extremely object side and has positive refractive power, and a second lens unit which is arranged on the image side of said first lens unit and has negative refractive power. In the recent years where compactness and enhancement of zooming ratios are demanded for zoom lens systems which are used widely, it is necessary for obtaining compactness to strengthen refractive powers of the lens units or shorten distance between the principal points of the lens units in the zoom lens systems of the type described above. Further, in order to enhance vari-focal ratios, it is effective to prolong the moving distance of the second negative lens unit which is used as the variator (the lens unit having the highest vari-focal function) and strengthen power of said lens unit. However, prolonging the moving distance of the lens unit is conflicting with compactness of lens systems. In order to design a compact zoom lens system having a high vari-focal ratio, it is therefore necessary to strengthen power of the second negative lens unit. When a field angle is widened at the wide position of a zoom lens system having a vari-focal ratio higher than 3, however, the second lens unit must have a very strong power, thereby making it difficult to correct aberrations. Since the second lens unit has the high vari-focal function, aberrations are varied remarkably by varying focal length and such variations cannot be corrected unless the aberrations produced by the second lens unit are not reduced.
The conventional zoom lens systems wherein the second lens unit is composed only of homogenous lens elements tend to have large negative Petzval's sums due to the negative power of the second lens unit, and produce especially spherical aberration and curvature of field which are remarkable on the positive sides. For this reason, it is obliged to compose the second negative lens unit of a very large number of lens elements to minimize the variations of aberrations to be caused by varying focal length. When the second negative lens unit is composed of a large number of lens elements, the principal point of the second lens unit is located within this lens unit, thereby making it impossible to reserve a distance between the principal points of the lens units arranged on the object side and the image side respectively of the second lens unit.
In the recent years, attention is attracted to the graded refractive index lenses (GRIN lenses) having refractive index varying depending on internal portions thereof. The GRIN lenses are classified into the so-called axial GRIN lens having refractive index varying in the direction along the optical axis and the so-called radial GRIN lens having refractive index varying in the direction perpendicular to the optical axis. Though the GRIN lenses are applied to optical systems for photographic cameras, no effective use has been established yet for these lenses.
As an example of the zoom lens systems using the GRIN lenses, there is known the lens system disclosed by Japanese Unexamined Published Patent Application No. 172110/61. This zoom lens system consists, in the order from the object side, of a first lens unit having positive refractive power, a second lens unit having negative refractive power and lens units arranged on the image side of the second lens unit, and uses a GRIN lens in the second negative lens unit. The GRIN lens used in this zoom lens system has curvature of the surface thereof which is lowered by sharing the power of the GRIN lens with the medium thereof. In other words, a portion of the power of the second lens unit (the variator) is shared as the power of the medium of the GRIN lens.
When the power of the second lens unit is partially shared as the power of a GRIN lens, however, the interior (medium) of the GRIN lens produces aberrations which are similar to those produced by imparting power to homogenous lens elements. The above-mentioned conventional zoom lens system in which negative power is imparted to the interior (medium) of the GRIN lens produces the aberrations similar to those produced by the homogenous lens elements. Accordingly, this zoom lens system is not suited for use as a zoom lens system having a high vari-focal ratio at the wide position where the second lens unit must have strong negative power.
Further, in order to design a zoom lens system consisting of a plural number of lens units and adapted to vary airspaces reserved between the lens units so as to have a high vari-focal ratio covering a range of wide angles, it is necessary to use a lens unit which has a high vari-focal function and strong negative power. When an attempt is made to compose this lens unit having the strong negative power only of homogenous lens elements, it is necessary to use a large number of lens elements, thereby making it impossible to design a compact zoom lens system.